Actually, if you're on the fence between a 5D4 and 90D, why are you even asking?
As mentioned, the 5D4 is a truly professional-grade camera, designed and built to be such.
It does not disappoint, and I'm still just trying to learn about what it's capable of.
Yes, it's been teased that there may be a MkV, but what the MkIV can do already
is absolutely phenomenal.
I have no experience with the 90D, but the 5D4 is all it's cracked up to be, and then some.
Image quality alone has been more than I'd hoped for, and I've gotten more leeway in
post-processing than was ever available from RAW files of my previous cameras.
My first use of it captured some images at 12800 ISO which are true keepers.
Set to Auto-ISO, I wasn't aware of the setting, but I'd have never attempted anything
over 3200 with my 50D or earlier cameras, knowing they could not have delivered.
Given the choice and budget allowance, I find no reason NOT to grab a 5D MkIV.